Method of making graphene and graphene devices

ABSTRACT

The present invention generally relates to a method of making graphene and graphene devices.

FIELD OF THE INVENTION

The present invention generally relates to a method of making graphene and graphene devices.

BACKGROUND OF THE INVENTION

Graphene is a substance composed of pure carbon, with atoms arranged in a regular hexagonal pattern similar to graphite, but in a one-atom thick sheet. It is very light, with a 1-square-meter sheet weighing only 0.77 milligrams. The structure of graphene is a single planar sheet of sp²-hybrid bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene is most easily visualized as an atomic-scale chicken wire made of carbon atoms and their bonds. The graphene atoms are arranged into a two-dimensional honeycomb structure with the crystalline or “flake” form of graphite consisting of many graphene sheets stacked together.

Graphene is about 100 times stronger than steel; conducts electricity better than copper; and is more flexible than rubber. It is touted as possible replacement for silicon in electronics.

Only identified in 2004, graphene is a single layer of tightly packed carbon atoms making it the thinnest material ever created and offering huge promise for a host of applications from information technology to energy to medicine. Graphene can be made by several methods such as scotch-tape or chemical ex-foliation, chemical vapor deposition (CVD) induced growth, graphite oxide reduction. The two primary methods of production are: chemical exfoliation and graphite oxide reduction. These methods unfortunately only produce small flakes of graphene (usually dispersed in a liquid medium). They also require use of aggressive solvents to break graphene oxide apart from the carbon source (such as graphite) and remove oxygen from graphene oxide to form graphene. Epitaxial Growth on a substrate produces larger graphene sheets (currently able to make up to 40″ square; and works by exposing CH₄ and H₂ to a substrate (such as copper foil) inside a high temperature furnace. This method requires etching of substrate to remove and transfer the graphene sheet. It is overall, a very costly and time consuming method to produce a large sheet of graphene.

In view of the above, it would be useful to be able to make graphene in a simpler, less costly way. It would also be useful to be able to make graphene directly on a device, thereby eliminating the need for transferring the graphene.

SUMMARY OF THE INVENTION

In an aspect, the present invention provides a novel graphene device precursor.

In another aspect, the present invention provides a novel method of making graphene.

In another aspect, the present invention provides a novel graphene device.

These and other aspects, which will become apparent during the following detailed description, have been achieved by the inventors' discovery of a new method of making graphene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the Raman spectra from a device that had been heated to 800° C. The spectra were taken at a point 3 μm from a Ni edge. These spectra (and the remainder described herein) are of the material that is located on the insulator layer.

FIG. 2 shows the Raman spectra from a device that had been heated to 800° C. The spectra were taken 50 μm from a Ni edge.

FIG. 3 shows the Raman spectra from a device that had been heated to 800° C. The spectra were taken 100 μm from a Ni edge.

FIG. 4 shows the Raman spectra from a device that had been heated to 600° C. The spectra were taken 5 μm from a Ni edge.

FIG. 5 shows the Raman spectra from a device that had been heated to 600° C. The spectra were taken 10 μm from a Ni edge.

FIG. 6 shows the Raman spectra from a device that had been heated to 700° C. The spectra were taken at a Ni edge.

FIG. 7 shows the Raman spectra from a device that had been heated to 700° C. The spectra were taken 50 μm from a Ni edge.

FIG. 8 shows the Raman spectra from a device that had been heated to 700° C. The spectra were taken 100 μm from a Ni edge.

FIG. 9 shows the Raman spectra from a device that had been heated to 700° C. The spectra were taken 200 μm from a Ni edge.

DETAILED DESCRIPTION OF THE INVENTION

In an aspect, the present invention provides a novel graphene device precursor, comprising:

-   -   an insulator layer, wherein at least the top portion of the         insulator layer is an electrical insulator;     -   a metal layer in contact with and covering part of the top of         the insulator layer;     -   a carbon layer in contact with the metal layer and the top of         the insulator layer;     -   an optional passivation layer located between the insulator         layer and the metal/carbon layers and in contact with and         covering a substantial portion of the top of the insulator         layer; and,     -   an optional metal adhesive layer located between the metal layer         and either the insulator layer or passivation layer, if present.

In another aspect, the insulator layer is a thermal oxide (thermal silicon oxide) layer (e.g., SiO₂/Si). For the thermal oxide wafer, at least the top portion of the wafer is SiO₂ (i.e., insulating). Typically, the top and bottom portions of thermal oxide wafers are SiO₂. Additional examples of insulators include crystalline quartz, sapphire, HBN, PBN, MgO, YSZ, and SiC. The thickness of the insulator layer (e.g., a 285 nm SiO₂/Si wafer) can vary depending upon the characteristics desired for the graphene device.

The metal layer covers only a part of the top of the insulator layer (and passivation layer, if present). In another aspect, the metal facilitates growth of graphene on the insulator layer (and passivation layer, if present). Examples of the types of metals that are useful are those having high carbon solubility (e.g., >1.5 atom % @1000° C.) and/or those having a crystal structure that acts as a graphene template. The metal layer can be one continuous piece (e.g., 2, 3, 4, 5 or more fingers connected by a perpendicular strip), multiple non-touching sections (e.g., 2, 3, 4, 5 or more non-connected strips or a plurality of dots or islands of metal), or even a combination (e.g., connected fingers and small non-connected dots or islands of metal located between the fingers). As an example, the metal can be present in a pattern that is useful to make an electronic device (e.g., an interdigital electrode (IDE) pattern). In another aspect, a sufficient amount of metal layer is present such that the graphene grown, in accordance with the method described herein, connects the different portions of metal (e.g., fingers, strips, dots, etc.).

In another aspect, the metal layer is Ni. Other examples of metals include Co, Re, Pd, and Pt. The thickness of the metal layer can vary depending upon the characteristics desired for the graphene device. Examples include from about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, to 200 nm.

It is not uncommon for metals such as Ni, Co, Re, Pd, and Pt to weakly adhere to an insulator layer (e.g., thermal oxide). Thus, in another aspect, a metal adhesion layer is present between the metal layer and the insulator layer (or between the metal and passivation layers, if the passivation layer is present). Examples of metal adhesion layers include Ti and Cr. Examples of the thickness of the optional adhesive layer include from about 1, 2, 3, 4, 5, 6, 7, 8, 9, to 10 nm. The metal adhesive layer is present in the same pattern as the metal layer (e.g., an IDE pattern).

The carbon layer is in contact with the metal layer and the top of the insulator layer (or passivation layer if present). Examples of the thickness of the carbon layer include from about 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, to 50 nm (or more if desired). In another aspect, the carbon layer is amorphous.

In the method described herein, heat is used to form graphene. However, some of the insulator layers described herein (e.g., thermal oxide) are not very stable at the upper temperature ranges used. One way to protect thermally unstable layers is to coat them with a passivation layer. Thus, in another aspect, a passivation layer is present. The passivation layer is located between the insulator layer and the metal/carbon layers and is in contact with and covering a substantial portion of the top of the insulator layer. The passivation layer is designed to cover a substantial portion of the insulator layer and thereby protect it. Examples of the thickness of the passivation layer include from about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, to 20 nm (or more if desired). The passivation layer is typically an oxide, such as Al₂O₃, HfO₂, Ta₂O₅, ZnO, TiO₂, and SiO₂.

Alternatively, the passivation layer is present, but only in the same pattern as the metal layer (and optional metal adhesive layer). In this aspect, the passivation layer is designed to protect the insulator layer from the metal layer during heating of the precursor.

In another aspect, the present invention provides a novel method of growing graphene, comprising:

-   -   (a) heating a graphene device precursor to a temperature         sufficient to initiate graphene formation; and,     -   (b) cooling the graphene device precursor.

Graphene refers to a layer of material, primarily comprising: graphene (a crystalline allotrope of carbon typically of a single atomic plane of graphite having a 2-dimensional hexagonal lattice structure of carbon atoms). The layer formed by the present invention is typically from 1, 2, 3, 4, 5, 6, 7, 8, 9, to 10 atomic layers in thickness.

In another aspect, the heating is conducted in a closed furnace. Other examples of heat sources include a substrate heater, microwave heater, RF heater, and UV heater.

In another aspect, the heating is conducted in a substantially oxygen-free atmosphere.

In another aspect, the heating is conducted in the presence of a substantially oxygen-free gas. An example of a gas is a hydrogen-containing gas (e.g., forming gas). Examples of gases include 95% Ar/5% H₂ and 95% N/5% H₂.

In another aspect, the precursor is heated to a temperature of about 400° C. Other examples of the temperature include from about 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, to 1000° C.

In another aspect, the temperature is maintained for about 1 minute. Other examples of the time the temperature is maintained include from about 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, to 55 minutes and from about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, to 5 hours.

In another aspect, the heating is conducted in the present of an O₂ scavenger. Examples of O₂ scavengers include Ti chips and a hydrogen-containing gas.

In another aspect, the heating is conducted in a vacuum. Examples of the pressure at which the heating is conducted include from about 500, 450, 300, 250, 200, 150, 100, 50, 25, 20, 10, 5, to 1 mT (mTorr or millitorr).

In another aspect, the cooling is conducted naturally. Natural cooling refers to turning off the power to the heat source (or removing the heat source) and letting the heat dissipate without further assistance.

In another aspect, the cooling is accelerated. As an example, accelerated cooling can be achieved by exposing the device to ambient atmosphere.

In another aspect, the present invention provides a novel graphene device, comprising:

-   -   an insulator layer, wherein at least the top portion of the         insulator layer is an electrical insulator;     -   a metal layer in contact with and covering part of the top of         the insulator layer;     -   a graphene layer in contact with the metal layer and the top of         the insulator layer;     -   an optional passivation layer located between the insulator         layer and the metal/carbon layers and in contact with and         covering a substantial portion of the top of the insulator         layer;     -   an optional metal adhesive layer located between the metal layer         and either the insulator layer or passivation layer, if present.

EXAMPLES

The following examples are meant to illustrate, not limit, the present invention.

Example 1

A small sample of a 285 nm SiO₂/Si wafer is cleaved via a diamond scroll to be used as the insulating layer.

The oxide surface (SiO₂) is then cleaned with acetone and methanol. The surface is further cleaned by reactive ion etching the surface in O₂ prior to metallization to remove any remaining organic substances.

An electron beam evaporation system (E-Beam) is then used to deposit a 200 nm thick Ni layer (the metal layer) onto the oxide surface in an electrode pattern.

A 10 nm layer of amorphous carbon is then deposited on the surface of the device (over the metal/oxide layers or metal/passivation layers) via filament carbon coater to complete a graphene device precursor (carbon/metal/insulator).

The graphene device precursor is loaded into a tube furnace along with boats of Ti chips. The tube furnace is pumped down to ˜3.5E-2 Torr and then backfilled with forming gas (95% Ar/5% H₂) to achieve ˜50 mT. The temperature of the tube furnace is run up to 800° C. for one hour and then allowed to cool naturally.

Example 2

Raman spectra obtained from a graphene device made according to Example 1 are shown in FIGS. 1 (taken 3 μm from a Ni edge), 2 (taken 50 μm from a Ni edge), and 3 (taken 100 μm from a Ni edge).

Example 3

Raman spectra obtained from a graphene device made according to Example 1, except that it was heated to 600° C. are shown in FIGS. 4 (taken 5 μm from a Ni edge) and 5 (taken 10 μm from a Ni edge).

Example 4

Raman spectra obtained from a graphene device made according to Example 1, except that it was heated to 700° C. are shown in FIGS. 6 (taken at a Ni edge), 7 (taken 50 μm from a Ni edge), 8 (taken 100 μm from a Ni edge), and 9 (taken 200 μm from a Ni edge).

Example 5

A small sample of a 285 nm SiO₂/Si wafer is cleaved via a diamond scroll to be used as the insulating layer.

A 5 nm passivation layer of Al₂O₃ is deposited via atomic layer deposition onto the SiO₂ (the top of the insulator layer).

The oxide surface (Al₂O₃) is then cleaned with piranha (3:1 H₂SO₄/H₂O₂). The surface is further cleaned by reactive ion etching the surface in O₂ prior to metallization to remove any remaining organic substances.

An E-Beam is used to deposit a 5 nm layer of Cr (the metal adhesive layer) in an interdigital electrode pattern.

The E-Beam is then used to deposit a 200 nm thick Ni layer (the metal layer) on the Cr interdigital electrode pattern.

A 10 nm layer of amorphous carbon is then deposited on the surface of the device via a filament carbon coater. Alternatively, the carbon may be sputtered onto the device.

The graphene device precursor (e.g., carbon/metal/adhesive/passivation/insulator) is loaded into a tube furnace along with boats of Ti chips. The tube furnace is pumped down to ˜3.5E-2 Torr and then backfilled with forming gas (95% Ar/5% H₂) to achieve ˜50 mT. The temperature of the tube furnace is run up to 800° C. for one hour and then allowed to cool naturally.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise that as specifically described herein. 

What is claimed is:
 1. A method of growing graphene using a precursor graphene device comprising: the precursor graphene device comprising: an insulator layer, wherein at least a top portion of the insulator layer is an electrical insulator; a metal layer in contact with and covering part of the top of the insulator layer; and a carbon layer in contact with the metal layer and the top of the insulator layer; said method comprising the steps of: (a) depositing the metal layer onto the insulator layer; (b) depositing carbon layer on the metal layer to complete the precursor graphene device; (c) loading the precursor graphene device into a tube furnace along with boats and oxygen free forming gas; and (d) heating the tube furnace to a temperature sufficient to initiate graphene formation; and (e) cooling the precursor graphene device.
 2. The method of claim 1, wherein: the heating is conducted in a substantially oxygen-free atmosphere.
 3. The method of claim 1, wherein: the heating is conducted in the presence of a substantially oxygen-free gas.
 4. The method of claim 1, wherein: the heating is conducted in the presence of a gas, comprising: 95% Ar/5% H₂.
 5. The method of claim 1, wherein: the precursor is heated to a temperature of from 400-1000° C.
 6. The method of claim 5, wherein: the temperature is maintained for 1 minute to 5 hours.
 7. The method of claim 1, wherein: the heating is conducted in a vacuum.
 8. The method of claim 7, wherein: the pressure at which the heating is conducted is from about 500-1 mT.
 9. The method of claim 1, further comprising a step of cleaning by reactive ion etching the insulator layer to remove organic substances.
 10. The method of claim 1, wherein the precursor graphene device further comprising a passivation layer located between the insulator layer and the metal/carbon layers and in contact with and covering a substantial portion of the top of the insulator layer.
 11. The method of claim 1, wherein the precursor graphene device further comprising a metal adhesive layer located between the metal layer and either the insulator layer or passivation layer. 